1. Field of the Invention
The present invention relates to an elastic surface-wave device comprising a substrate and a surface acoustic wave generator which is provided on a surface of the substrate and generates surface acoustic waves on the surface. More specifically, the present invention relates to a spherical elastic surface-wave device in which the substrate has a region being formed by at least a part of a spherical surface in the surface and circularly continuous, and the surface acoustic wave generator generates surface acoustic waves propagating in a direction along which the surface region of the substrate is continuous.
2. Description of the Related Art
An elastic surface-wave device which generates surface acoustic waves on a substrate and receives the surface acoustic waves generated on the substrate, is well known.
The surface acoustic wave is an elastic wave which intensively propagates almost all of its energy in a surface of a material, is different from a regular longitudinal wave or a transverse wave known as a bulk wave. As the surface acoustic wave, a Rayleigh wave, a Sezawa wave, a pseudo-Sezawa wave, a Love wave and others can be exemplified, and such waves can exist on a surface of an anisotropic substance.
In a conventional elastic surface-wave device, a pair of comb-like electrodes are provided on a flat piezoelectric material arranged on a flat substrate. When a high-frequency electric current is supplied to one comb-like electrode, surface acoustic waves are generated from the piezoelectric material in a direction along which one comb-like electrode is aligned on the substrate. The other comb-like electrode is arranged on the substrate in a moving direction of the surface acoustic waves generated from one comb-like electrode, and receives the surface acoustic waves.
The elastic surface-wave device is used for a delay line, an oscillation device and a resonance device for a transmitter, a filter for selecting a frequency, a chemical sensor, a biosensor, or a remote tag and the like.
A width of the surface acoustic waves in a direction orthogonal to a propagation direction of the surface acoustic waves generated by one of the pair of comb-like electrodes is generally equal to a length for which a plurality of electrode pieces of one comb-like electrode and a plurality of electrode pieces of the other comb-like electrode are opposed to each other. In the specification of the present application, this length is referred to as an electrode width.
In the elastic surface-wave device, in order to increase the accuracy of a resonance frequency of the surface acoustic wave transmitting between the pair of comb-like electrodes, it is desirable to reduce propagation losses which are generated when the surface acoustic waves are propagated between the pair of comb-like electrodes, as much as possible.
However, in the usual elastic surface-wave device, since the surface of the piezoelectric material to which the pair of comb-like electrodes are provided and the surface of the substrate are flat, the surface acoustic waves generated by one comb-like electrode are diffused in a direction orthogonal to the propagation direction of the surface acoustic waves on the surface and are weakened, when the surface acoustic waves are propagated on the flat surface toward the other comb-like electrode. Therefore, the propagation losses of the surface acoustic waves can not be reduced, and hence there is a limit to the increase of the performance in the elastic surface-wave device.
This invention is derived from the above-described problems, it is an object of the present invention to provide an elastic surface-wave device of greatly increased performance, compared with a prior art elastic surface-wave device, and which is compact.
To achieve the above-mentioned object, an elastic surface-wave device according to the present invention, comprises: a substrate; and a surface acoustic wave generator which is provided on a surface of the substrate and generates surface acoustic waves on the surface,
wherein the substrate has a region which is configured by at least a part of a spherical surface in the surface and circularly continuous, and
wherein the elastic surface-wave generator is provided in the surface region of the substrate and generates surface acoustic waves in such a manner that the surface acoustic waves propagate along the surface region of the substrate only in a direction in which the surface region of the substrate is circularly continuous, without being diffused in a direction which crosses the continuous direction.
In the elastic surface-wave device according to the present invention and characterized by this configuration, the surface acoustic wave generator which is provided in the surface region of the substrate being configured by at least a part of a spherical surface and circularly continuous, generates surface acoustic waves so that the surface acoustic waves propagate in a direction in which the surface region of the substrate is circularly continuous on an outer surface of the substrate. Then, the surface acoustic waves propagate only in the continuous direction without being diffused in the crossing direction, along the surface region. Thus, the surface acoustic waves can propagate along the surface region without being diffused for a circumferential distance of the surface region, so that they can propagate infinitely.
Accordingly, a performance of the present invention can be greatly improved as compared with the conventional elastic surface-wave device. Further, since the substrate has in the surface thereof the surface region which is configured by at least a part of the spherical surface and circularly continuous, the device of the present invention can be compact.
In the elastic surface-wave device according to the present invention and characterized by being configured as described above, the substrate may be formed of a non-piezoelectric material. In this case, the surface acoustic wave generator includes: a piezoelectric material film provided in the surface region of the substrate; and an oscillator which is provided on a surface of the piezoelectric material film and generates surface acoustic waves in the continuous direction by applying an electric field to the piezoelectric material film.
The oscillator may include a comb-like electrode connected to a high-frequency power supply.
The substrate may be formed of a piezoelectric material. In this case, the surface acoustic wave generator may be provided with an oscillator which is provided in the surface region on the surface of the substrate and generates surface acoustic waves in the continuous direction by applying an electric field to the surface region of the substrate.
The oscillator may include a comb-like electrode connected to a high-frequency power supply.
When the oscillator includes the comb-like electrode connected to the high-frequency power supply, it is preferable that an arrangement cycle of a plurality of electrode pieces of the comb-like electrode is set to be not more than {fraction (1/10)} of a radius of the spherical surface of the substrate.
A wavelength of the surface acoustic waves generated by the oscillator is not a cycle of natural oscillation of the entire substrate but is substantially equal to the arrangement cycle of a plurality of the electrode pieces of the comb-like electrode.
When the oscillator includes the comb-like electrode connected to the high-frequency power supply, it is preferable that a length (electrode width) for which a plurality of the electrode pieces of the comb-like electrode are opposed to each other is set to be not more than a half of the diameter of the spherical surface of the substrate and not less than {fraction (1/100)} of the radius of the spherical surface.
A length of the comb-like electrode arranged in the surface region and an electric circuit pattern and the like attached thereto in a direction orthogonal to the continuous direction must be not more than a half of the circumferential length of the spherical surface of the substrate. Therefore, it is reasonable that a length (electrode width) for which a plurality of electrode pieces of the comb-like electrode are opposed to each other is not more than a half of the diameter of the spherical surface of the substrate. Further, when the length (electrode width) for which the electrode pieces are opposed to each other is not more than {fraction (1/100)} of the radius of the surface region of the substrate, the surface acoustic waves generated in the comb-like electrode are diffused in the direction orthogonal to the continuous direction while they are propagated in the continuous direction of the surface region. Then, when the surface acoustic waves diffused in the direction orthogonal to the continuous direction are inputted to the comb-like electrode, the comb-like electrode is influenced by an obstacle existing in a region in which the surface acoustic waves are diffused, and, for example, a frequency characteristic of the comb-like electrode may be possibly adversely affected.
In fact, it is preferable that a wavelength parameter (a circumferential length/a wavelength of surface acoustic waves in the continuous direction of the spherical surface) is 100 to 800 and a length (electrode width) for which a plurality of the electrode pieces of the comb-like electrode are opposed to each other in the orthogonal direction is equal to or more than a collimate angle (angle at which a collimated beam can be obtained).
It is preferable that the arrangement cycle of a plurality of the electrode pieces of the comb-like electrode is not more than {fraction (1/10)} of the radius of the spherical surface.
Moreover, it is preferable that each distance between a plurality of the electrode pieces of the comb-like electrode is not more than {fraction (1/10)} of the radius of the spherical surface.
In the present invention, the xe2x80x9csubstrate having a surface including a region which is formed by a part of a spherical surface and circularly continuousxe2x80x9d, of course, includes a substrate having a spherical shape and also includes a barreled substrate in which regions other than the above-described continuous region is cut off from the spherical shape, or a substantially disc-like substrate having a peripheral surface convexly curved toward the outside. In addition, a substrate having a spherical, barreled or substantially disc-like cavity is also included.
Therefore, the substrate may include a cavity including an inner surface having a region which is formed by at least a part of a spherical surface and circularly continuous, and an opening for causing the inside of the cavity to communicate with the outside of the substrate.
In such a case, the surface acoustic wave generator is provided in the continuous region of the inner surface of the cavity.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.